This renewal application, for years 23-27, is to continue studies of the neuroendocrinology of major depression. The studies planned herein build upon more than 20 years of systematic exploration of the HPA axis in major depression, supported by this grant. In previous years, we conducted several large studies of patients with major depression vs normal controls, the results of which have led to our current focus on the hypothalamo-pituitary-adrenal cortical (HPA) axis. This axis is mildly to moderately hyperactive in 30-50 percent of depressed patients, most likely on the basis of increased corticotropin releasing hormone (CRH) and arginine vasopressin (AVP) secretion from the hypothalamus. Our most recent studies have revealed an unexpected and very interesting finding, the HPA axis responds differently to cholinergic challenge not only in depressed patients vs normal controls but also in women vsmen. Our extension of this to the study of laboratory rats has revealed similar findings between female and male animals. This is an emerging area of CNS neuropharmacology, with only a few prior studies that have considered neuroendocrine responses, other than the hypothalamo-pituitary-gonadal axis, by sex. Specific aims include: 1) Continued analysis of data from earlier studies. 2) Continued detailed study of sex differences in cholinergic regulation of the HPA axis in patients with major depression vs normal controls, in depressed patients during illness vsafter successful treatment, and in women us men. This specific aim forms the major thrust of this renewal application. 3) Completion of HPA axis studies at the adrenal and pituitary levels. 4) Continuation of animal studies of sexual diergism of cholinergic regulation of the HPA axis in female and male rats. The human studies will be used to inform planning of the studies in laboratory animals, and vice versa. The significance of the planned studies lies in their furthering our understanding of HPA axis regulation by CNS cholinergic mechanisms in major depression, as well as what we hypothesize to be adaptive changes to increased CNS stimulation of the axis at the pituitary and adrenal levels. In particular, understanding sex differences in cholinergic regulation of the HPA axis may help elucidate biological mechanisms underlying the greater incidence of major depression in women vsmen.